As the technology develops, people have been gradually placed importance on how to convert green energy into usable power for human beings. There are many green energy conversion device have been provided in the market, and one of them is a power conversion device applied in a photo-voltaic (PV) power system. Such a power conversion device can be referred to as a PV inverter.
Refer to both of the FIGS. 1A and 1B. FIG. 1A is a circuit diagram showing a conventional power conversion device 100. FIG. 1B is a schematic diagram showing the function blocks of the power conversion device in FIG. 1A. The power conversion device 100 outputs an alternating current iout according to a direct-current (DC) voltage Ed and a grid voltage Vs.
The power conversion device 100 includes four active switch elements T21 to T24 at its output side. The four active switch elements T21 to T24 receive a control signal with relatively high frequency, i.e. a signal with a frequency higher than the frequency of the grid voltage, such as 50 to 60 Hz. In general, the control signal has a relatively high frequency, such as a frequency higher than 10 KHz, so as to avoid noise audible to the human ears. However, under the control of the control signal with relatively high frequency, the active switch elements T21 to T24 are switched on and off frequently, thus increase the power dissipation and decrease the conversion efficiency of the power conversion device 100. Therefore, how to enhance the power efficiency of the power conversion device is still a subject of the industrial endeavor.